This application is based upon and claims priority from prior French Patent Application No. 99-12380, filed Oct. 5, 1999, the entire disclosure of which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to the field of semiconductor packages containing an integrated-circuit chip and more particularly to the process of forming electrical connections on semiconductor packages with solder drops and solder balls.
2. Description of Related Art
Presently, semiconductor packages have, on the surface, metal electrical-connection regions or pads. These metal electrical-connection regions are generally made of copper and are covered with an anti-oxidation first metal layer. The first metal layer generally consisting of nickel (Ni) or cobalt (Co) or a nickel-cobalt alloy. A second metal layer generally made of gold (Au) or of palladium (Pd), is formed on top of the first metal layer. Along with the second metal layer, a metal solder drop or solder ball is formed.
The solder drop or solder ball is generally made of an alloy containing tin (Sn) combined with lead (Pb), silver (Ag) and/or copper (Cu). The first layer provides an anti-diffusion barrier in order to prevent direct contact between the copper layer and the solder drop. The solder drops are generally used to make an electrical connection with another circuit, for example a printed circuit.
Although these surface metal connection packages are useful, they are not without their problems. One problem is mechanical stress. When the packages are subjected to mechanical stresses or to cycles of temperature variations, or when the packages age, many times some of the solder drops become detached.
The problem of the solder drops becoming detached is due in part to the fact that, during the operation of soldering the solder drops to the connection regions, the metal constituting the second metal layer dissolves in the solder drop. This produces a tin-gold or tin-palladium precipitate. Over time, this precipitate migrates and reforms a layer on the first layer. This precipitate layer weakens and embrittles the connection or the interface between the solder drop and the first metal layer.
Accordingly, a need exists to overcome the above problems with the prior art and to provide a process and semiconductor package which reduces the embrittlement problem so as to limit the risk of the solder drops or balls de-bonding.
The present invention reduces the embrittlement phenomenon of the prior art so as to limit the risk of the solder drops or balls de-bonding.
The process for producing electrical connections on the surface of a semiconductor package containing an integrated-circuit chip and having metal electrical-connection regions on the surface of the package consists of: covering these metal electrical-connection regions with a first metal layer forming an anti-diffusion barrier; covering this first layer with an anti-oxidation second metal layer; and depositing a metal solder drop or solder ball on the second metal layer.
According to the invention, the solder drop comprises an addition of metal particles in suspension which contain at least one of the metals of the first metal layer, so as to produce a precipitate comprising these additional metal particles and at least partly the metal of the second metal layer the precipitate remaining in suspension in the solder drop.
According to one embodiment of the invention, the sum of the surface areas of the particles may be at least equal to half the area of contact between the solder drop and the connection region.
According to another embodiment of the invention, the sum of the volumes of the particles may be between 0.5 and 5 percent of the volume of the solder drop.
According to another embodiment of the invention, the first metal layer may advantageously comprise nickel (Ni) and/or cobalt (Co), the second metal layer may advantageously comprise gold (Au) or palladium (Pd) and the solder drop may advantageously comprise tin (Sn) and an addition of particles containing nickel (Ni) and/or cobalt (Co).